Field effect transistors are well known in the art, including various gating techniques. An enhancement mode FET conducts current between its drain and source in response to gate voltage. Because of the capacitance that exists between the gate, source and drain of the FET, any change in the gate voltage is achieved only through an attendant movement of charge to and from the FET gate region. The speed with which a FET can be turned on and off is dependent upon the speed with which the charge can be stored in and removed from the gate capacitance. Some gating circuits are able to supply sufficient current to charge the gate rapidly to attain fast turn-on. The present invention relates to this latter type of gating circuit.
The invention provides FET gate drive circuitry including in combination a current source for supplying constant current to maintain requisite FET gate voltage, and a bypass capacitor connected to the control resistance means of the current source for momentarily short-circuiting the latter at turn-on. A momentarily higher current is supplied by the current source to the FET gate until the bypass capacitor is charged, whereafter lower level constant current is supplied.